1. Field of the Invention
The present invention relates to a control system for controlling a control amount via a movable mechanism that can vary the control amount within a predetermined control range.
2. Description of the Related Art
Conventionally, there has been proposed a control system for controlling the lift of an intake valve (hereinafter referred to as “the valve lift”) of an internal combustion engine in Japanese Laid-Open Patent Publication (Kokai) No. 2003-254100. The engine has a variable valve lift mechanism provided for each cylinder. The variable valve lift mechanism continuously varies the valve lift between a predetermined maximum value and a predetermined minimum value, and includes a drive shaft connected to a crankshaft and a control shaft parallel to the drive shaft. On the drive shaft, there are provided a swinging cam and an link arm which are connected to a rocker arm on the control shaft.
The control shaft is rotatably supported by a bearing, and has a pin radially outwardly extending from the peripheral surface thereof at a location close to the bearing. The bearing is formed with a protrusion, and when the control shaft rotates in a predetermined direction, it is brought into contact with the protrusion whereby the rotation of the control shaft is stopped. Further, the control shaft is provided with a rotation drive mechanism that drives the control shaft for rotation, and the rotation drive mechanism includes an electric motor and a gear mechanism. When the rotation drive mechanism drives the control shaft for rotation, the relative angular positional relationship between the swinging cam and the link arm, and the rocker arm, whereby the valve lift is changed. In particular, when the pin of the control shaft is held in contact with the protrusion of the bearing, the valve lift is held at the predetermined minimum value.
The control system includes various sensors, a controller to which are connected these sensors and the electric motor, and so forth. The controller determines an operating region of the engine based on detection signals from the sensors, and controls the valve lift via the variable valve lift mechanism according to the operating region, whereby the amount of intake air supplied to the engine is controlled. Moreover, when the controller determines that the engine is in a low-load operating region, such as idling, the controller causes the control shaft to be rotated until the pin is brought into contact with the protrusion, whereby the valve lift of each cylinder is controlled to the predetermined minimum value.
According to the above conventional control system, during the control of the valve lift, when it is determined that the engine is in a low-load operating region, the control shaft is driven for rotation until the pin of the control shaft is shifted to a position where it is in contact with the protrusion of the bearing, so that when the rational speed of the control shaft is high, the pin and the protrusion may be deformed due to an impact applied thereto when the pin is brought into contact with the protrusion. To avoid this problem, there can be envisaged e.g. a method of reducing the rotational speed of the control shaft so as to reduce the impact, and a method of providing shock absorbers for the pin and the protrusion. When the rotational speed of the control shaft is reduced, however, it takes a longer time to drive the pin to the position of abutment with the protrusion, i.e. to control the valve lift to the minimum value, and hence it takes a longer time to control the intake air amount to the proper value. As a result, the controllability of the intake air amount is lowered, so that when the engine is in a low-load operating region, e.g. during idling, the engine speed can be made unstable. On the other hand, when the shock absorbers are provided for the pin and the protrusion, the manufacturing costs are increased due to the provision of these additional members, and since space for the shock absorbers has to be secured, this degrades the degree of freedom in design of the engine.